The present invention relates to a linear, low voltage operation, push-pull amplifier for use in submarine cable systems.
In submarine cable systems, the power is supplied to the repeater-amplifiers from the shore stations, and dc repeater voltages are in series. For a long cable system with a multitude of repeater-amplifiers, it is essential that the dc voltage required by each repeater-amplifier be as small as possible in order to keep terminal dc voltages at levels that are compatible with reliable corona free performance.
It is accordingly a primary object of the invention to operate a repeater-amplifier with low supply voltage.
And a related object is to provide a push-pull amplifier circuit that operates with a lower supply voltage than the conventional Class A, Class B or Class AB push-pull amplifiers.
Since a multitude (e.g., 1500) of repeater-amplifiers are coupled in tandem in a submarine cable system, it is imperative that each amplifier be as linear as possible. The distortion (e.g., crossover distortion) created in any given amplifier will be exacerbated in those that follow.
It is a further object of the invention, therefore, to provide an amplifier circuit that has a high degree of linearity and is thus substantially free of distortion.
The patent to S. T. Brewer, U.S. Pat. No. 3,676,613, issued July 11, 1972, discloses a high efficiency, Class B, push-pull amplifier for use in submarine cable systems. Like Class B push-pull amplifiers in general, the amplifier of Brewer achieves a high degree of power efficiency, but this is obtained at a sacrifice in amplifier linearity. As pointed out by Brewer, linearity can be improved by utilizing feedforward techniques, but this introduces added circuit complexity and costs. Also, as with Class B push-pull amplifiers generally, the collector voltage provided by the voltage supply source must be of a magnitude at least as great as the peak of the largest signal to be handled. Yet, as previously pointed out, it is important in submarine cable systems that the voltage required by each repeater be as small as possible.
Accordingly, a further object of the invention is to provide a linear amplifier, for use in submarine cable systems, which requires low supply voltage and which utilizes simple and reliable components.